1. Field of the Invention
The present invention relates to an inductor layout and in particular to an inductor layout using step symmetry for the inductors and mirror symmetry for the rest of the circuit.
2. Description of the Related Art
Inductors can be used in many types of circuits. For example, in radio frequency (RF) circuits, inductors can be used in voltage-controlled oscillators (VCOs), low-noise amplifiers, and passive-element filters. In general, an inductor is a passive electrical device that stores energy in a magnetic field.
An inductor is physically formed using one or more turns of wire and has two terminals. FIG. 1 illustrates a layout of an exemplary spiral inductor 100 including an outer terminal 101 and an inner terminal 102. Inductor 100 is formed in a square pattern, which can provide more inductance for a given surface area and can be easily fabricated using standard integrated circuit techniques. In one technique, inductor 100 can be built on a substrate (e.g. silicon) using at least two metal layers. Specifically, a first metal layer can be used to form spiral coil 103 whereas a second metal layer can be used to form an underpass contact 104. Underpass contact 104 can be connected to inner terminal 102 using a via (not shown).
RF circuits are increasingly using differential techniques to achieve enhanced linearity, dynamic range, and output power. In a differential circuit, signals with equal magnitude and opposite phase are transmitted and processed using two sets of components. Of importance, both sets of components are ideally symmetrical with respect to each other. This symmetry can be used to reject common-mode coupling, which can otherwise result in interference and undesirably affect signal processing.
Mirror symmetry for a layout of a differential circuit is commonly used because of its effectiveness in providing identical parasitic wiring capacitances on both sides of the differential circuit, thereby simplifying computations taking into account these capacitances. For example, FIG. 2 illustrates a schematic of an exemplary bandpass filter 200 including devices, e.g. inductors 201 and capacitors 202 and 203, that can be formed using mirror symmetry (e.g. using inductor layouts 201′ and 201″).
However, in a differential circuit that includes inductors, current flows in opposite directions (indicated by arrows 206 and 207). Using the well known “right hand rule”, the magnetic field generated by the inductor using layout 201′ would be going into the plane including the inductor, which is shown by symbol 208. Unfortunately, the magnetic field using layout 201″ would also be going into the plane including the inductor, which is shown by symbol 209 (i.e. the magnetic fields would be positioned in the same direction). A sensitive circuit positioned near this inductor pair may be adversely affected by this combined magnetic field.
In contrast, if step symmetry is used (for example, if both inductors of a differential circuit are instantiated using layout 201′), the net far field transmitted is zero, since the magnetic field generated by one inductor effectively cancels the magnetic field generated by the other. Sensitive circuits may thus be placed in proximity to inductor pairs in step symmetry with no substantial degradation.
Similarly, a magnetic field couples as a common-mode signal when inductor step symmetry is used. For example, FIG. 3 illustrates a schematic of bandpass filter 200 that can be formed using step symmetry (e.g. using inductor layout 201′). A magnetic field would therefore create a common-mode current in this inductor pair and can thus advantageously substantially cancel the effect of the coupled field.
In contrast, a magnetic field would couple as a differential signal if inductors in a differential circuit are instantiated using mirror symmetry (e.g. using inductor layouts 201′ and 201″). Deleterious interference from a nearby circuit producing a magnetic field may result in this inductor configuration.
Unfortunately, using step symmetry makes balancing of parasitic wiring capacitances between the two sides of the differential circuit difficult. Therefore, a need arises for a technique and layout that provide the benefits of both mirror and step symmetries.